The present invention is related to the field of optical fiber networks, and more particularly, to multiple fiber Bragg gratings which can be used in optical fiber networks.
In fiber optics, semiconductor lasers provide the only practical source of light signals for optical fiber networks. Narrow linewidth semiconductor laser sources are highly desirable in applications such as WDM (Wavelength Division Multiplexing) networks. In a WDM network, the wavelength of optical signals is used to direct the signals from its source to the desired destination. Hence the narrow linewidths provided by the narrow linewidth semiconductor laser sources allow a large number of communication channels to operate over the network. Narrow linewidth light sources are required, for instance, in a Dense WDM network standard with channels having a separation of 100 GHz frequency (or 0.8 nm wavelength) which the International Telecommunications Union (ITU) has proposed.
One type of laser source is found in U.S. Pat. No. 5,699,377, entitled NARROW LINEWIDTH, STABILIZED SEMICONDUCTOR LASER SOURCE to J. J. Pan, which discloses a narrow linewidth semiconductor laser diode source having a laser diode and a fiber Bragg grating. The laser diode has first and second facets from which output light is emitted. A first end of the fiber Bragg grating is located near the second facet to receive output light from the laser diode. The fiber Bragg grating has a very narrow reflection band about a selected wavelength and reflects output light in a selected narrow linewidth back into the laser diode through its second facet. The output light emitted from the first facet has a very narrow linewidth about the selected linewidth. The entire disclosure of this patent is incorporated herein by reference.
Because laser diode sources are expensive, it is desirable to produce a laser diode source with multiwavelength outputs. To provide multiwavelength outputs of different narrow linewidths, a plurality of fiber Bragg gratings which reflect light at the different wavelengths can be provided near the second facet of the laser diode. Cascading multiple, spaced fiber Bragg gratings in series is more practical than constructing the fiber Bragg gratings in parallel because of the problem of interfacing the input and output light in a parallel structure. Parallel fiber Bragg gratings also require multiple laser diodes. A cascading multiple fiber Bragg gratings structure has certain disadvantages. These include long physical length which requires a long package size and insertion loss due to fusion of the fiber Bragg gratings in series. Moreover, it is difficult to make a temperature compensated package.